Obturator Nerve Block

ANATOMY

The obturator nerve (L2-4) descends towards the pelvis from the medial border of the psoas major muscle. After passing through the obturator foramen, the obturator nerve enters medial aspect of the thigh, and divides into an anterior branch between the adductor longus and brevis muscles and a posterior branch between the adductor brevis and magnus muscles. The posterior branch usually gives off an articular branch to the knee joint.

The obturator nerve provides sensory innervation to a relatively small area on the medial aspect of the knee. However, blockade of the obturator nerve can decrease opioid requirements and improve analgesia following knee replacement surgery when combined with femoral and sciatic nerve blocks.   AB, AL & AM = adductor brevis, longus & magnus muscles   FN = femoral nerve   ON = obturator nerve   PECT = pectineus muscle   SAR = sartorius muscle

SCANNING TECHNIQUE

• Position the patient supine with the leg to be blocked slightly externally rotated.
• Expose the groin and the medial aspect of the proximal thigh.
• After skin and transducer preparation, place a linear transducer with the appropriate frequency range (10-12 MHz) in the inguinal crease and scan slightly distally in the upper medial thigh.
• Optimize machine imaging capability; select appropriate depth of field (usually within 2-4 cm), focus range and gain.

Figure A. A high frequency linear transducer is placed on the medial side of the left thigh below the inguinal crease.

Figure B. Note the left leg is slightly externally rotated.

ANATOMICAL CORRELATION

The Obturator Nerve in the Proximal Thigh

AB = adductor brevis muscle   AL =adductor longus muscle   AM = adductor magnus muscle   PECT = pectineus muscle   White arrowhead = anterior branch of obturator nerve   Yellow arrowhead = posterior branch of obturator nerve

NERVE LOCALIZATION

• Perform a systematic anatomical survey on the medial aspect of the proximal thigh.
• The two branches of the obturator nerve are expected to lie within the fascial planes between the adductor muscles.
• Identify the femoral vein below the inguinal crease and then move the linear transducer medially.
• Identify the pectineus and adductor longus muscles located medially. The anterior branch of the obturator nerve (usually a hyperechoic structure) can be found between the adductor longus (more superficial) and brevis muscles.
• The posterior branch (often hyperechoic) is found between the adductor brevis (more superficial) and magus muscles
• Trace the course of the obturator nerve proximally and distally for a short distance.
• Low current electrical stimulation of the obturator nerve will elicit adductor muscle contraction and aid nerve localization.

OUT OF PLANE NEEDLE INSERTION APPROACH

• Ultrasound guided obturator nerve block is considered a INTERMEDIATE skill level block. It can be challenging to image the obturator nerve because of its small size.
• Insert a 5 cm 22 G insulated needle perpendicular to the transducer and the ultrasound beam (see picture below).
• Identify the anterior and posterior branches of the obturator nerve lying in the fascial plane between the adductor longus and brevis muscles and the adductor brevis and magnus muscles respectively.
• Confirm the identity of the branches of the obturator nerve by electrical stimulation and differentiate between the anterior and posterior divisions.

Figure A. Out of plane needle insertion on the medial side of the left leg below the inguinal crease.

Figure B. Needle (arrow) approaching the anterior branch of the obturator nerve (arrowhead).

LOCAL ANESTHETIC INJECTION

• Inject 5-10 mL of local anesthetic in each of the 2 intermuscular fascial planes. Observe local anesthetic distention of the intermuscular planes and local anesthetic surrounding the hyperechoic nerve structures.
• Scan proximally and distally to assess the extent of local anesthetic spread.

Injection to the Posterior Branch of Obturator Nerve

AB, AL & AM = adductor brevis, longus and magus muscles   Arrowhead = posterior branch of obturator nerve

AB, AL & AM = adductor brevis, longus and magus muscles   Arrow = needle tip   Arrowhead = posterior branch of obturator nerve   LA = local anesthetic

CLINICAL PEARLS

Needle Insertion and Local Anesthetic Injection

• Visualization of the needle tip with the out of plane approach can be challenging because of the depth required to reach the posterior branch.
• The “hydro location” technique can be very useful to localize the needle tip.
• The “hydro dissection” technique is also useful to “dissect” out the intermuscular fascial plane before local anesthetic injection.

IMAGE GALLERY

Pre-injection Scan   The obturator nerve (arrowheads) appears strongly hyperechoic immediately below the inguinal crease.   AB = adductor brevis muscle externus muscle   AL = adductor longus muscle   PECT = pectineus muscle

Anatomical Correlation

Obturator Nerve in the Proximal Thigh Immediately Below the

AB = adductor brevis muscle   AL = adductor longus muscle   OE = obturator externus muscle   PECT = pectineus muscle

Detailed anatomy within the scanned area (red dotted rectangle)   The obturator nerve (arrowheads) is located between the pectineus (PECT) and adductor brevis (AB) muscles.   AL = adductor longus muscle   FV = femoral vein   OE = obturator externus muscle

Pre-injection Scan   As the transducer is moved more distally away from the inguinal crease, the obturator nerve is seen to divide into the anterior (A) and posterior (B) branches.

Post-injection Scan   A small amount of local anesthetic (LA) has been injected through the needle. Note the needle is visualized as a hyperechoic dot (arrow) in cross section.

Post-injection Scan   Local anesthetic (LA) is now visualized around the nerve (arrowhead) and has distended the intermuscular fascial plane.

VIDEO GALLERY

Obturator Nerve Block (Out of Plane Approach)

SELECTED REFERENCES

• Saranteas T, Paraskeuopoulos T, Alevizou A, Kouskouri A, Zogojiannis J, Anagnostopoulou S, Chantzi C. Identification of the obturator nerve divisions and subdivisions in the inguinal region: a study with ultrasound. Acta Anaesthesiol Scand 2007;51:1404-1406.
• Fujiwara Y, Sato Y, Kitayama M, Shibata Y, Komatsu T, Hirota K. Obturator nerve block using ultrasound guidance. Anesth Analg 2007;105:888-889.
• Helayel P E, da Conceicao D B, Pavei P, Knaesel J A, Oliveira Filho G R. Ultrasound-guided obturator nerve block: a preliminary report of a case series. Reg Anesth Pain Med 2007;32:221-226.
• Soong J, Schafhalter-Zoppoth I, Gray A T. Sonographic imaging of the obturator nerve for regional block. Reg Anesth Pain Med 2007;32:146-151.